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So how come calling Pluto a planet is so goofy? page 2
Pluto: Not demoted, just reassigned, by Greg Black, of the University of Virginia

 Pluto, imagined here with its moon Charon, was demoted recently to dwarf planet. But there is more to the Plutonic 'neighborhood' than meets the eye.
RON MILLER/ATOMIC ART
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Date published: 5/16/2010

continued

Obvious criteria such as setting a minimum size for a planet turn out to be quite arbitrary. When a glass or coffee mug is dropped, one knows that it shatters into many pieces. There may be just a couple of large pieces, many smaller ones, and many more of the smallest fragments. The formation of the solar system might be viewed in much the same way with a distribution of different-size objects orbiting the sun. We call the largest ones the planets, but there is really no clear demarcation between what we'd call big versus small, making that cutoff somewhat arbitrary.

The International Astronomical Union has members from 90 countries and among its purposes is to oversee the official naming of celestial objects. For example, the naming of Pluto must be agreed upon simply to ensure that everyone is talking about the same object. There were several proposals for planet definitions, including grandfathering Pluto's planet status, or expanding the definition to include 12 planets or possibly even more that might be discovered. In a conference held in Prague in 2006, these proposals were debated and eventually the final version was passed by a vote of attending members. Science does not typically operate by public vote in this manner, but at issue was not the terms of a scientific theory but rather nomenclature.

THREE REQUIREMENTS

A definition of a planet would preferably mirror the practical usage of the term. Everyone agrees that the Earth is a planet, and so are Jupiter, Saturn, etc. But the moon isn't, and comets aren't. So any definition should preserve those common-sense notions also.

The first part of the approved definition is perhaps the simplest requirement: A planet must orbit the sun. However, that is not sufficient as other objects, such as comets, asteroids, and even dust, orbit the sun.


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POPULAR PLUTO FAILED THE FINAL TEST

Gregory Black is a planetary scientist at the University of Virginia.